1. Field of the Invention
The present invention relates to a composition which can be used to manufacture an electron transport layer using a wet coating method at a low temperature, an electron transport layer manufactured using the same, and an organic electroluminescent device including the electron transport layer, and more particularly, to an organic electroluminescent device including an electron transport layer manufactured by coating and drying a composition for forming an electron transport layer which is a solution formed of a mixture of Al2O3 particles and a titanium precursor and thus, having low operating voltage and improved lifetime property.
2. Description of the Related Art
Organic electroluminescent devices, which are active display devices, use the recombination of electrons and holes in a fluorescent or phosphorescent organic compound thin layer (hereinafter, referred to as ‘organic layer’) to emit light when current is applied thereto. Organic electroluminescent devices are lightweight, have wide viewing angles, produce high-quality images, and can be manufactured using simple processes. Organic electroluminescent devices also can produce moving images with high color purity while having low consumption power and low voltage. Accordingly, organic electroluminescent devices are suitable for portable electronic applications.
In general, an organic electroluminescent device includes an anode, a hole transport layer, an emission layer, an electron transport layer, and a cathode sequentially stacked on a substrate. The hole transport layer, the light emitting layer, and the electron transport layer are organic membranes formed of organic compounds.
The organic electroluminescent device may operate as follows. When a voltage is applied between the anode and the cathode, holes emitted by the anode move to the light emitting layer via the hole transport layer. Electrons are emitted by the cathode and move to the light emitting layer via the electron transport layer. In the light emitting layer, the carriers recombine to produce excitons. The excitons radiatively decay, emitting light corresponding to a band gap of a material of the light emitting layer.
Materials that can be used for forming the light emitting layer of the organic electroluminescent device are divided, according to emission mechanism, into fluorescent materials using singlet excitons and phosphorescent materials using triplet-state excitons. The light emitting layer is formed by using such fluorescent materials or phosphorescent materials themselves or by doping such fluorescent materials or phosphorescent materials on appropriate host materials. When electrons are excited, singlet excitons and triplet excitons are generated in a host in the generation ratio of 1:3 (Baldo, et al., Phys. Rev. B, 1999, 60, 14422).
When fluorescent materials are used to form the light emitting layer in the organic electroluminescent device, triplet excitons that are generated in the host cannot be used. However, when phosphorescent materials are used to form the light emitting layer, both singlet excitons and triplet excitons can be used, and thus, an internal quantum efficiency of 100% can be obtained (see Baldo et al., Nature, Vol. 395, 151-154, 1998). Accordingly, the use of phosphorescent materials brings higher light emitting efficiency than use of fluorescent materials.
However, even though the light emitting efficiency improves by using such phosphorescent materials, adequate level of the light emitting efficiency required in a light emitting device is yet to be provided, and thus, various improvement methods of such light emitting devices have been devised.
One improvement method is inducing multiple light emission to improve the light emitting efficiency, instead of inducing single light emission by interposing a charge generation layer within the light emitting layer. Also, the light emitting efficiency can be improved by improving electrical and physical properties of metals and organic membranes to control interfacial property. However, the processes of such methods are complicated and thus, are too expensive or an adequate level of the light emitting efficiency required in a light emitting device can not be provided.
In addition, based on a method of improving charge delivery capacity using specific polymeric materials, the process of bonding holes and electrons to form activated molecules is optimized and the positions where these activated molecules emit light are uniformly dispersed, thereby improving the light emitting efficiency.
In relation to carrier injection, when the center of the light emitting layer is excited, densities of holes and electrons are balanced. This is the most important key in bringing high efficiency of a device.
For example, when the electron transport layer (ETL) is interposed between the organic light emitting layer (EML) and the cathode, most electrons emitted by the cathode move toward the anode to the EML to recombine with holes. However, when the hole transport layer (HTL) is interposed between the anode and EML, electrons that move into the EML are blocked by the interface of the HTL and thus cannot move further toward the anode and instead are kept in the EML, thereby improving recombining efficiency.
That is, when the ETL and the HTL are interposed between the EML and the electrode, the following improvements can be obtained: quantum efficiency increases; operating voltage lowers by injecting carriers by the second operation which passes through the transport layer instead of direct injecting; the light emitting efficiency improves, since electrons/holes injected into the light emitting layer are blocked by the hole/electron transport layer when moving to the anode/electrode, thereby enabling recombination control; and quenching (quenching is a phenomenon in which light emission of materials decreases as light emitting molecules become closer) can be prevented since singlet exciton generated by recombining of electrons and holes is formed on a boundary between the electrode and the light emitting layer.
In addition, a hole injection layer (HIL) may be interposed between the anode and HTL for a more efficient injection of holes, wherein the HIL is formed of an organic material having a work function of 5.0-5.2 eV which is decided by considering a work function of ITO anode electrode (4.7-5.0 eV) and Ionization Potential (IP) of HTL in order to lower an energy barrier when injecting holes to HTL from the anode.
Moreover, salts such as LiF and NaCl are further stacked between the cathode electrode and ETL to act as a buffer layer to improve efficiency of the device, here, differently from a concept of the HIL, metals having high reduction such as lithium and sodium are doped by co-evaporation near the interface of the cathode and thus, an electron injection barrier is lowered, thereby reducing a required operating voltage.
Electrodes highly affects to intensity of light emitted and efficiency of the organic electroluminescent device, this is because the intensity of light emitted depends on current injected into a light emitting device. Light emission efficiency in an Mg electrode is 50 times higher than in an Al electrode. Accordingly, Al electrodes need more buffer layers to inject electrons more efficiently.
When inorganic compounds are used in the ETL, both thermal and chemical stability are improved compared to when organic compounds are used and thus, when the device operates for long time, electron transport capacity does not change, thereby showing stable performance of the device for a long time.
An Al cathode and the light emitting layer have low efficiency. For example, when Al2O3 having a suitable thickness is interposed as the electron transport layer between the Al cathode and the light emitting layer of Alq3, current injection and light emission output is highly improved. It arises from improved electron tunneling and removing exciton quenching and carrier trapping gap state on the Alq3/Al interface [APL, 70, 1233(1997)].
However, when only Al2O3 is used in the ETL, improvement in the light emitting efficiency is limited. Therefore, development of more efficient ETL materials is required.